P05 –Comparative exploration of KEA3 function in PMF partitioning and photoprotection

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The chloroplast PMF is produced via light-driven proton-coupled electron transport across the thylakoid membrane. This generates both a membrane potential (∆ψ) and a proton concentration gradient (∆pH). Their contribution to PMF is governed in part by thylakoid ion transport processes. The ∆pH component of the PMF or more specifically, the lumenal proton concentration determines the fraction of light energy that is used for photochemistry. Our data from Arabidopsis thaliana (Arabidopsis) show that the thylakoid K+-exchange antiporter 3 (KEA3) plays a fundamental role for the flexibility of vascular plant photosynthesis during light fluctuations. KEA3 can sense the energy state of the chloroplast stroma and adjusts levels of the photoprotective energy dependent quenching (qE) mechanism in response. If the energy state is low, KEA3 gets activated to decrease the proton concentration of the lumen and shuts off qE. While KEA3 activity affects PMF composition and decreases the ∆pH component, it appears to have little effect on PMF size and thus its energetic function. Previously, we could demonstrate that de-regulated KEA3 activity can result in transiently higher CO2 fixation rates. This proposes KEA3 regulation as a putative target for enhancing photosynthesis under the dynamic light conditions found in nature. An in-depth understanding of KEA3 regulation and its interaction with the complex network of photosynthetic reactions will be pivotal to further explore this avenue. Within the framework of GoPMF, we seek to extent our knowledge of KEA3 function in PMF partitioning and its regulation via a comparative approach: (i) By measuring KEA3 activity in the unicellular alga Chlamydomonas reinhardtii (Chlamydomonas), we aim to understand evolutionary effects on KEA3 function and regulation, (ii) by comparing stromal energy dynamics between Arabidopsis and Chlamydomonas, we aim to characterize how environmental changes affect the chloroplast energy state and relate this to KEA3 activity , (iii) by determining KEA3 regulating factors, we will link changes in the stromal energy state with KEA3 activity and finally (iv) by expressing modified KEA3 versions in planta, we will confirm these links experimentally and use our knowledge on PMF modulation by KEA3 for attempts to improve photosynthetic performance and growth (see also Graphical Abstract P05). Our work will be integral for the aim GoPMF 2 and additionally contribute to aims 1, 3, 4 and 5.